Lathes of this type, which are known, for example, from DE-A-30 35 451 or DE-A-35 14 069, usually have a horizontal machining plane. However, lathes of this type can, in principle, also be constructed such that the machining plane extends vertically or is inclined relative to a vertical plane by, for example, 30.degree. (so-called sloping bed machines). The machining plane is hereby to be understood as that plane which is defined by the spindle axis and the direction of the so-called X-axis of the lathe because the cutting edges of tools of the two tool carriers, which are located in operating position, are displaced in this plane when they are moved forward towards the spindle axis, parallel to the X-axis in a radial direction relative to the spindle axis, in order to reduce the diameter of a workpiece to be machined.
The known lathes resulting from the publications cited above have a machine base with a horizontal upper side, on which bar-like guideways (Z-guides) for the bed-side carriages of two cross-slide systems, each of which bears a tool turret, are attached in front of and behind the spindle axis, these guideways extending parallel to the spindle axis direction (direction of the so-called Z-axis of the lathe). Since each of the two cross-slide systems of these known lathes has, apart from the lower slide or bed-side carriage displaceable in the direction of the Z-axis, an upper slide displaceable horizontally and at right angles to the spindle axis direction, i.e. in the direction of the X-axis of the lathe, the machining plane of these known lathes extends horizontally and parallel to the support and attachment surfaces for the Z-guides for the bed-side carriages, these surfaces being formed by the upper side of the machine base. This constructional principle has a whole series of disadvantages: Since tools held by the tool carriers must be movable in the machining plane, i.e. at a level above the upper side of the machine base predetermined by the position of the spindle axis, the bed-side carriages can only have the shape of a relatively thin plate, on which, for example, the drive motors for driving the upper slides of the two cross-slide systems must be mounted. In addition, the slide guideways, above all the Z-guides for the bed-side carriages mounted on the machine base, are located in the range of flight of the chips or shavings resulting during the machining. This can cause considerable problems which can only be solved with complex telescopic covers made of metal sheets or in an unsatisfactory manner by wiper means. The chip problem is particularly serious when a so-called tailstock, the headstock of a workpiece spindle and/or a backrest are also guided on the Z-guides for at least one of the bed-side carriages since these elements and the bed-side carriages must be displaceable independently of one another and able to be brought close to one another without any space therebetween which precludes the use of a telescopic cover.
Finally, an energy supply line to the bed-side carriage of the cross-slide system located in front of the spindle axis also poses problems in these known lathes because a line of this type is located in the flight range of the chips resulting from workpiece machining and extends transversely through the operating space of the lathe.
DE-39 21 649-A discloses a lathe of the type mentioned at the outset, in which some of the disadvantages mentioned above can be avoided or diminished. In this known lathe, a slide bed extending in the direction of the Z-axis is attached to an upper side of a machine base, this upper side being inclined forwards (towards an operator) and downwards relative to the horizontal. Z-guides for a bed-side carriage of an upper cross-slide system displaceable in the direction of the Z-axis are attached to the top of the slide bed while a front side of the slide bed, which is inclined downwards and rearwards relative to a vertical plane extending in the direction of the Z-axis, bears Z-guides for the bed-side carriage of a lower cross-slide system. The bed-side carriage of the upper cross-slide system has, roughly, the shape of a relatively thin plate with a lower side which has a saw-tooth profile when seen in cross section at right angles to the Z-axis and with an upper side, on which a Y-slide displaceable in horizontal direction at right angles to the Z-axis is guided. This Y-slide has a vertical front side extending parallel to the Z-axis and an X-slide displaceable in vertical direction is guided thereon. This X-slide bears a tool turret having an indexing axis oriented in the direction of the Z-axis. The bed-side carriage of the lower cross-slide system has an approximately triangular cross section (vertically to the Z-axis) with a front side oriented vertically and parallel to the Z-axis. An X-slide displaceable in vertical direction is guided on this front side and this X-slide likewise bears a tool turret having an indexing axis parallel to the Z-axis.
In this known lathe, the horizontally extending spindle axis of a workpiece spindle, which again defines the direction of the Z-axis, is arranged between the superposed tool turrets and the machining plane extends vertically. Due to the inclination of the front side of the slide bed rearwards and downwards, it is scarcely possible for chips or shavings to remain on the Z-guides for the bed-side carriage of the lower cross-slide system. However, an energy supply line to the bed-side carriage of the lower cross-slide system must be located in the flight range of the chips and extend in the direction of the Z-axis through the operating space of the lathe. Moreover, the bed-side carriages have to be relatively flat because the Y-slide of the upper cross-slide system is mounted on the upper bed-side carriage and, for reasons of stability, the spindle axis cannot be arranged at a large horizontal distance in front of the slide bed. This means that the thickness of the lower bed-side carriage measured in the direction of the Y-axis is limited.
All the known lathes described in the above have an additional, serious disadvantage. In order to understand this, it must first of all be explained what forces occur during an unintentional collision between a rotating chuck of the workpiece spindle, which holds a workpiece to be machined, and a tool or the tool carrier (tool turret) bearing this tool: In relation to the spindle axis, these forces are oriented tangentially and vertically to the machining plane and are correspondingly high due to the peripheral speed of the chuck which is very high in comparison with the displacement speed of a slide.
In the known lathes described in the above, which have bed-side carriages displaceable in a horizontal plane and a horizontal machining plane, these collision forces extend at right angles to the horizontal upper side of the bed bearing the two bed-side carriages and therefore result in permanent damage to the lathes since, in this direction, neither the Z-guides guiding the bed-side carriages nor the headstock mounting the workpiece spindle and placed on the horizontal upper side of the bed can avoid these forces. Such collisions do, however, also lead to permanent damage in the lathe according to DE-39 21 649-A, namely for the following reason: In this known lathe, parts of the Z-guides for the two bed-side carriages are guide bars which extend in the direction of the Z-axis and have transversely thereto a cross section which either corresponds exactly or approximately to a flat rectangle. When seen in the direction of the Z-axis, the broader transverse surfaces of these bars extend, for each respective bed-side carriage, parallel to one another, namely for the upper bed-side carriage at an angle forwards and downwards and for the lower bed-side carriage at an angle rearwards and downwards. A collision, as described in the above, results in any case in considerable forces occurring at each of these bars and these forces act in a direction, in which neither these bars nor the headstock can move.
The same applies to the forces which occur during a collision between a tool or tool holder of a tool turret just being rotated for the purpose of indexing and a workpiece, a workpiece spindle or its headstock.